Toxicity The safe upper limit for vitamin B6 has been set at 100 mg/d, although no adverse effects have been associated with high intakes of vitamin B 6 from food sources only. When toxicity occurs, it causes a severe sensory neuropathy, leaving patients unable to walk. Some cases of photosensitivity and dermatitis have also been reported. Folate, Vitamin B12 See Chap. 90. Vitamin C Both ascorbic acid and its oxidized product dehydroascorbic acid are biologically active. Actions of vitamin C include antioxidant activity, promotion of nonheme iron absorption, carnitine biosynthesis, the conversion of dopamine to norepinephrine, and the synthesis of many peptide hormones. Vitamin. | Chapter 071. Vitamin and Trace Mineral Deficiency and Excess Part 6 Toxicity The safe upper limit for vitamin B6 has been set at 100 mg d although no adverse effects have been associated with high intakes of vitamin B6 from food sources only. When toxicity occurs it causes a severe sensory neuropathy leaving patients unable to walk. Some cases of photosensitivity and dermatitis have also been reported. Folate Vitamin B12 See Chap. 90. Vitamin C Both ascorbic acid and its oxidized product dehydroascorbic acid are biologically active. Actions of vitamin C include antioxidant activity promotion of nonheme iron absorption carnitine biosynthesis the conversion of dopamine to norepinephrine and the synthesis of many peptide hormones. Vitamin C is also important for connective tissue metabolism and crosslinking proline hydroxylation and it is a component of many drug-metabolizing enzyme systems particularly the mixed-function oxidase systems. Absorption and Dietary Sources Almost complete absorption of vitamin C occurs if 100 mg is administered in a single dose however only 50 or less is absorbed at doses 1 g. Enhanced degradation and fecal and urinary excretion of vitamin C occur at higher intake levels. Good dietary sources of vitamin C include citrus fruits green vegetables especially broccoli tomatoes and potatoes. Consumption of five servings of fruits and vegetables a day provides vitamin C in excess of the RD A 90 mg d for males and 75 mg d for females. In addition approximately 40 of the . population consumes vitamin C as a dietary supplement in which natural forms of vitamin C are no more bioavailable than synthetic forms. Smoking hemodialysis pregnancy and stress . infection trauma appear to increase vitamin C requirements. Deficiency Vitamin C deficiency causes scurvy. In the United States this is seen primarily among the poor and elderly in alcoholics who consume 10 mg d of vitamin C and also in individuals consuming macrobiotic diets. In addition to .
